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Circulation of blood to the head and neck

Arterial supply

The paired arteries supplying the head and neck are the common carotid arteries and the vertebral arteries (Figs 5.32, 5.34).

Figure 5.32

Main arteries of the left side of the head and neck.

Figure 5.34

The right vertebral artery.

Carotid arteries

The right common carotid artery is a branch of the brachiocephalic artery. The left common carotid artery arises directly from the arch of the aorta. They pass upwards on either side of the neck and have the same distribution on each side. The common carotid arteries are embedded in fascia, called the carotid sheath. At the level of the upper border of the thyroid cartilage each divides into an internal carotid artery and an external carotid artery.

The carotid sinuses are slight dilations at the point of division (bifurcation) of the common carotid arteries into their internal and external branches. The walls of the sinuses are thin and contain numerous nerve endings of the glossopharyngeal nerves. These nerve endings, or baroreceptors, are stimulated by changes in blood pressure in the carotid sinuses. The resultant nerve impulses initiate reflex adjustments of blood pressure through the vasomotor centre in the medulla oblongata (p. 89).

The carotid bodies are two small groups of specialised cells, called chemoreceptors, one lying in close association with each common carotid artery at its bifurcation. They are supplied by the glossopharyngeal nerves and their cells are stimulated by changes in the carbon dioxide and oxygen content of blood. The resultant nerve impulses initiate reflex adjustments of respiration through the respiratory centre in the medulla oblongata.

External carotid artery

(Fig. 5.32)This artery supplies the superficial tissues of the head and neck, via a number of branches:

The superior thyroid artery supplies the thyroid gland and adjacent muscles.

The lingual artery supplies the tongue, the lining membrane of the mouth, the structures in the floor of the mouth, the tonsil and the epiglottis.

The facial artery passes outwards over the mandible just in front of the angle of the jaw and supplies the muscles of facial expression and structures in the mouth. The pulse can be felt where the artery crosses the jaw bone.

The occipital artery supplies the posterior part of the scalp.

The temporal artery passes upwards over the zygomatic process in front of the ear and supplies the frontal, temporal and parietal parts of the scalp. The pulse can be felt in front of the upper part of the ear.

The maxillary artery supplies the muscles of mastication and a branch of this artery, the middle meningeal artery, runs deeply to supply structures in the interior of the skull.

Internal carotid artery

This is a major contributor to the circulus arteriosus (circle of Willis) (Fig. 5.33), which supplies the greater part of the brain. It also has branches that supply the eyes, forehead and nose. It ascends to the base of the skull and passes through the carotid foramen in the temporal bone.

Figure 5.33

Arteries forming the circulus arteriosus (circle of Willis) and its main branches to the brain.

Circulus arteriosus (circle of Willis)

The greater part of the brain is supplied with arterial blood by an arrangement of arteries called the circulus arteriosus or the circle of Willis (Fig. 5.33). Four large arteries contribute to its formation:

the two internal carotid arteries and the two vertebral arteries (Fig. 5.34). The vertebral arteries arise from the subclavian arteries, pass upwards through the foramina in the transverse processes of the cervical vertebrae, enter the skull through the foramen magnum, then join to form the basilar artery. The arrangement in the circulus arteriosus is such that the brain as a whole receives an adequate blood supply when a contributing artery is damaged and during extreme movements of the head and neck.

Anteriorly, the two anterior cerebral arteries arise from the internal carotid arteries and are joined by the anterior communicating artery.

Posteriorly, the two vertebral arteries join to form the basilar artery. After travelling for a short distance the basilar artery divides to form two posterior cerebral arteries, each of which is joined to the corresponding internal carotid artery by a posterior communicating artery, completing the circle. The circulus arteriosus is therefore formed by:

2 anterior cerebral arteries 2 internal carotid arteries

1 anterior communicating artery 2 posterior communicating arteries 2 posterior cerebral arteries

1 basilar artery.

From this circle, the anterior cerebral arteries pass forward to supply the anterior part of the brain, the middle cerebral arteries pass laterally to supply the sides of the brain, and the posterior cerebral arteries supply the posterior part of the brain.

Branches of the basilar artery supply parts of the brain stem.

Venous return from the head and neck

The venous blood from the head and neck is returned by deep and superficial veins.

Superficial veins with the same names as the branches of the external carotid artery return venous blood from the superficial structures of the face and scalp and unite to form the external jugular vein (Fig. 5.35).

Figure 5.35

Veins of the left side of the head and neck.

The external jugular vein begins in the neck at the level of the angle of the jaw. It passes downwards in front of the sternocleidomastoid muscle, then behind the clavicle before entering the subclavian vein.

The venous blood from the deep areas of the brain is collected into channels called the dural venous sinuses.

The dural venous sinuses of the brain (Figs 5.36 and 5.37) are formed by layers of dura mater lined with endothelium. The dura mater is the outer protective covering of the brain (p. 146). The main venous sinuses are listed below:

The superior sagittal sinus carries the venous blood from the superior part of the brain. It begins in the frontal region and passes directly backwards in the midline of the skull to the occipital region where it turns to the right side and continues as the right transverse sinus.

The inferior sagittal sinus lies deep within the brain and passes backwards to form the straight sinus.

The straight sinus runs backwards and downwards to become the left transverse sinus.

The transverse sinuses begin in the occipital region. They run forward and medially in a curved groove of the skull, to become continuous with the sigmoid sinuses.

The sigmoid sinuses are a continuation of the transverse sinuses. Each curves downwards and medially and lies in a groove in the mastoid process of the temporal bone. Anteriorly only a thin plate of bone separates the sinus from the air cells in the mastoid process of the temporal bone. Inferiorly it continues as the internal jugular vein.

Figure 5.36

Venous sinuses of the brain viewed from the right.

Figure 5.37

Venous sinuses of the brain viewed from above.

The internal jugular veins begin at the jugular foramina in the middle cranial fossa and each is the continuation of a sigmoid sinus. They run downwards in the neck behind the sternocleidomastoid muscles. Behind the clavicle they unite with the subclavian veins, carrying blood from the upper limbs, to form the brachiocephalic veins.

The brachiocephalic veins are situated one on each side in the root of the neck. Each is formed

by the union of the internal jugular and the subclavian veins. The left brachiocephalic vein is longer than the right and passes obliquely behind the manubrium of the sternum, where it joins the right brachiocephalic vein to form the superior vena cava (Fig. 5.38).

Figure 5.38

The superior vena cava and the veins that form it.

The superior vena cava, which drains all the venous blood from the head, neck and upper limbs, is about 7 cm long. It passes downwards along the right border of the sternum and ends in the right atrium of the heart.